1. Field of the Invention
The present invention relates to a process of efficiently producing a hydrophobic organosilica sol comprising a colloidal silica stably dispersed in a hydrophobic organic solvent, a part of silanol groups on the surface of the silica particles being silylated. This process is useful for the production of a silica sol comprising a colloidal silica dispersed in ketones or esters.
2. Description of the Related Art
U.S. Pat. No. 2,433,776 discloses a technique of adding a water-soluble solvent to an aqueous silica sol containing an inorganic salt to salt out, extracting, and forming an organosilica sol. For example, Example 2 of this U.S. patent describes adding sulfuric acid to sodium silicate aqueous solution, adding n-propanol to the resulting mixture to separate an organic layer, dehydrating to a certain extent, removing a salt, and further dehydrating the organic layer, thereby obtaining propanol-dispersed sol. Example 6 of the U.S. patent also describes that the propanol sol can be substituted with butyl acetate.
U.S. Pat. No. 2,786,042 discloses a process of producing a hydrophobic silica sol having an average particle size of 10 to 150 nm in which silyl groups having 1 to 20 carbon atoms are bonded to at least 5% of surface Si atoms by reacting hydrocarbon-substituted silanol on the surface of particle. Halogen, alkoxy and sodium compounds are exemplified as a compound for forming hydrocarbon-substituted silanol. For example, an alkali siliconate aqueous solution is added to an aqueous silica sol, the resulting mixture is neutralized with formic acid, and t-butyl alcohol and sodium chloride are added to the neutralized mixture to salt out, thereby obtaining an aqueous organosilica sol.
U.S. Pat. No. 2,801,185 discloses a silica colloid redispersible in an organic solvent, which has an average particle size of 5 to 150 nm, is rendered hydrophobic with alcohol or organosilyl group chemically bonded to silica surface, and has hydroxyl group surface of 10 m.sup.2 or less per 1 g as measured by methyl red adsorption method. For example, when dimethyl dichlorosilane is added to a silica sol having triethyl phosphate as a dispersion medium, and after reaction, hydrogen chloride (hydrochloric acid), a solvent, excess dimethyl dichlorosilane and the like are distilled off, a solid dispersible in benzene or chloroform is obtained.
JP-A-57-196717 describes that a silica powder having esterified surface thereof and dispersible in an organic solvent is obtained by heating a silica sol dispersed in an alcohol having 2 to 18 carbon atoms to 170 to 300.degree. C., and then distilling off the alcohol. For example, silica powder redispersible in, for example, methyl ethyl ketone is obtained by heating ethyl alcohol silica sol to 200.degree. C., and then removing a liquid phase.
JP-A-58-145614 describes that a silica powder redispersible in an organic solvent, having silyl groups having 1 to 36 carbon atoms bonded to a surface of colloidal silica particles in an amount of 1 to 100/10 nm.sup.2 is obtained by adding a silylating agent to an organosilica sol having a water content of 10% or less to conduct reaction, and then distilling off the solvent. Chlorosilane compounds, alkoxysilane compounds, silazane compounds, hydroxysilane compounds and the like are exemplified as the silylating agent. It is described therein that the amount of bonded alkyl groups necessary for obtaining redispersibility varies depending on a carbon atom number of the silyl group, and when the carbon atom number is 3, the amount is 25/10 nm.sup.2 or more. For example, trimethylchlorosilane is added to n-butyl alcohol silica sol to conduct reaction, and the resulting reaction mixture is dried under reduced pressure to obtain hydrophobic silica powder. This powder has bonded silyl groups of 32.8 per 10 nm.sup.2, and when a dispersion having 10 g of this powder dispersed in 100 cc of toluene is centrifuged, the amount of the powder precipitated is 0.1 g. Further, it is described that a medium for the organosilica sol having a water content of 10% or less, which is a raw material, is a hydrophilic organic solvent, and a hydrophobic organic solvent which is compatible each other can be used in combination. It is also described that an alcohol having 3 or less carbon atoms directly reacts with the silylating agent, and therefore is not preferable as the hydrophilic solvent.
JP-A-3-187913 describes that a trimethylsilylating agent is added to a methanol sol obtained by hydrolyzing alkyl silicate in methanol, in an amount of 5 mol % or more per 1 mol of SiO.sub.2 in terms of silica to conduct reaction, and excess trimethylsilylating agent and dispersion solvent are distilled off to obtain a silica powder having excellent dispersibility, surface of which being silylated. For example, 20 mol %, per 1 mol of SiO.sub.2 in terms of silica, of methoxytrimethylsilane is added to silica particle methanol dispersion obtained by hydrolyzing tetramethylsilane in methanol in the presence of aqueous ammonia, and excess silylating agent is recovered, followed by drying, to obtain hydrophobic silica powder.
U.S. Pat. No. 5,651,921 (corresponding to JP-A-4-108606 and 4-170313) discloses a process of producing a water repellent silica sol, which comprises silylating silica surface of a non-polar organic solvent-dispersed silica sol. It is disclosed therein that the non-polar organic solvent-dispersed silica sol is obtained by adding the non-polar organic solvent to an alcohol-dispersed silica sol, and substituting a solvent by distillation. This U.S. patent also discloses a process of producing a water repellent silica sol by a process of adding a non-polar organic solvent, a cationic surface active agent and a silylating agent to an aqueous silica sol to form an emulsion, and then removing water from the emulsion by azeotropic dehydration.
JP-A-6-298519 discloses a process of producing a water repellent silica sol by adding a non-polar organic solvent, a water-soluble alcohol, a cationic surface active agent and a silylating agent to an aqueous silica sol to separate a water layer, and then dehydrating an organic layer under reflux.
European Patent Publication 768351 (corresponding to International Patent Publication WO96/34063) discloses a process of producing a dispersion of a solvent-dispersed inorganic oxide sol, comprising removing water contained in an aqueous inorganic oxide sol under azeotropic distillation with an azeotropic agent for water, and surface treating with a silane coupling agent, and also a process of substituting the dispersion of the solvent-dispersed inorganic oxide sol with other solvent. Silica sol and the like are listed as the inorganic oxide. Monoalkyltrimethoxysilane compounds, monoalkyltriethoxysilane compounds, dialkyldimethoxysilane compounds, and the like are listed as the silane coupling agent. Water-soluble alcohols and the like are listed as the azeotropic solvent. Alcohol, methyl ethyl ketone, methyl isobutyl ketone, dimethyl acetoamide and the like are exemplified as a solvent which can further substitute the dispersion of a solvent-dispersed inorganic oxide sol. Experimental example 17 of the European patent publication discloses a process of producing a silica sol having an average particle size of 30 nm dispersed in cyclohexane by substituting a solvent of an aqueous sol having an average particle size of 20 nm with isopropyl alcohol, surface treating with methyltrimethoxysilane, and further substituting the solvent with cyclohexane.
A process comprising substituting an aqueous silica sol with a water-soluble solvent, and further substituting with a hydrophobic organic solvent has conventionally been known as a process of forming an organosilica sol dispersed in a hydrophobic organic solvent using an aqueous silica sol as a raw material. However, the organosilica sol obtained by this process is unstable because of low hydrophilicity of a solvent which ultimately becomes a dispersion medium. For this reason, agglomeration of particles occurs in the solvent substitution step or during storage of a sol obtained, and viscosity increase or agglomeration precipitation may often occur. Thus, a sol having high concentration, low viscosity and storage stability over long period of time is not yet obtained.
Further, to disperse the sol in hydrophobic organic solvent a process of subjecting silica surface to hydrophobic treatment has been proposed.
One of conventional hydrophobic treatment methods is a method of heating a sol in the presence of excess alcohol to esterify silanol groups on the silica particle surface. This reaction requires high temperature, so that where alcohol having a low boiling point is used, it is required to heat in, for example, an autoclave. Alternatively, a method of heating a sol in high boiling point alcohol may be used. However, the high boiling alcohol has the problem that it is difficult to remove excess alcohol after reaction. The hydrophobic silica obtained by this esterification method has the disadvantage that hydrophobicity tends to be lost by hydrolysis of alkoxy group.
Another conventional method for hydrophobic treatment is a method of treating silica surface with a silylating agent or a silane coupling agent. Compared with the esterification method, this method can conduct reaction under relatively mild conditions, and some methods are proposed.
A method of treating a silica sol with trialkoxysilane compounds or dialkoxysilane compounds to improve dispersibility in an organic solvent or paint is widely conducted. However, considerably large amount of alkoxysilane compounds must be used in order to disperse the sol in hydrophobic organic solvent, and therefore this method is not always efficient. Further, since trialkoxysilane compounds or dialkoxysilane compounds are polyfunctional, not only reaction with a silica sol but also condensation reaction of alkoxysilane compounds each other tends to occur, and because of the condensation reaction gelation or cross linking between particles may also occur. Thus it is difficult to obtain hydrophobic organosilica sol having good dispersibility. In addition, it is difficult to remove a condensate of residual alkoxysilane compounds which are not bonded to a silica sol, and the condensate of residual alkoxysilane compounds may adversely affect in using the hydrophobic organosilica sol to a paint or the like.
Chlorosilane compounds also have conventionally widely used for hydrophobic treatment. Those compounds have high reactivity, but had the problem in corrosion of an apparatus due to hydrochloric acid by-produced, agglomeration of colloidal silica in reaction, and the like. Further, it is difficult to completely remove hydrochloric acid from a silica sol after treatment.
Further, where alkali siliconate compounds are used, it is difficult to remove an alkali by-produced, and where silazane compounds are used, it is also difficult to remove ammonia by-produced. Such by-products may impair the performance of a sol which is the final product.
Further, the conventional process of conducting a hydrophobic reaction of silica particles, drying the product, pulverizing the product, and dispersing the resulting powder in other organic solvent is an advantageous process for removing an alcohol, hydrochloric acid, ammonia and the like by-produced by hydrophobic reaction. However, agglomeration and bonding of particles tend to occur in drying the particles. In order to prevent this, a covering proportion of a silica particle surface with hydrophobic groups must be increased, but it was even still difficult to completely prevent the agglomeration and bonding. For example, when a hydrophobic powder is redispersed, a precipitate is caused in an amount of 1% or more per powder, as described in the examples of JP-A-58-145614. Therefore, in order to prevent bonding between particles during drying the silica particle surface must be covered in high proportion. As a result, the sol obtained does not have bonding property even if dried, and also it was difficult to further react unreacted silanol groups with other reagent such as a silane coupling agent.
A process is known comprising surface treating with a silylating agent or a silane coupling agent and then conducting a solvent substitution. However, in the convention process, viscosity of a sol after surface treatment may increase, and thus it was difficult to conduct a solvent substitution of post-treatment in high concentration. Further, agglomerate may be formed or gelation may occur by polymerization of a silane coupling agent, during the solvent substitution.
In a process of directly silylating a silica particle surface in a non-polar organic solvent as disclosed in U.S. Pat. No. 5,651,921, an aggregation of a non-polar organic solvent-dispersed silica sol tends to occur, and a silica sol of low viscosity sufficiently dispersed even after surface treatment cannot be obtained. In addition, a silica concentration cannot be increased, and such a silica sol is only applied to a limited use.
Even in any of processes, dispersion of silica particles is not sufficient. For this reason, addition of a large amount of a surface treating agent or reaction at high temperature is required, and this is not an industrially efficient process.